Vinyl plastisol resins are extremely important from a commercial standpoint but they must have a high plasticizer capacity and have increased flexibility, workability and distensibility. Usually, these resins are made by emulsion polymerization processes. In emulsion polymerization of vinyl and vinylidene halides to make polymers and copolymers thereof, relatively large quantities of emulsifier or soap are needed which leads to difficulties in processing the finished dispersion or paste vinyl resins.
In emulsion polymerization, the finished product is a polymer latex which, theoretically, can be coagulated, filtered to remove the polymer particles and then can be dried. However, this is difficult to do without forming agglomerates of polymer which cannot be disintegrated without using harsh grinding procedures which in turn adversely affects the quality of the polymer. If the resin is used as is, the agglomerates are excessively large and detract from the usefulness of the resin when subsequently making vinyl resin plastisols. As a result, it has been the common practice in the industry to take the polymer latex having polymer particles of the proper predetermined size and spray-dry the latex. However, this procedure presents problems, one of which is that when spray-drying, all of the emulsifier or soap, initially added to the polymerization mixture, is deposited on the polymer particles. The large quantity of soap so deposited usually shows up as a cloudiness, or a turbidity, when the resin or polymer is fused with a plasticizers in making a plastisol. Also, the large amount of soap present adversely affects the viscosity of the plastisol in that it produces a higher yield value than that desired. Further, the heat stability of the plastisol is affected by the soap since the same discolors at processing temperatures producing off-colored products.
While many attempts have been made to remove the soap from vinyl dispersion resins by simple washing, most have run into various kinds of technical problems. Also, the various proposals have been expensive and complicated. As a result, little has been done to remove the soap after the spray-drying operation.
Further, and importantly, it is known that spray-drying it is an expensive process with a high expenditure of energy but is has been required for production of high quality fine particle size vinyl dispersion resins. A great deal of work has been done in attempts to develop processes for eliminating the spray-drying step but problems have been encountered and the resulting resins have been of lower or poorer quality than conventional spray dried resins. One method is to use coagulating agents on the vinyl polymer latex to precipitate the polymer and then filter the same by usual methods prior to spray-drying or other means of drying. Most effective coagulation aids or agents have residues that degrade the quality of the polymer or resin. Drying of such materials, after coagulation and filtration, by most conventional methods results in hard lumpy particles and satisfactory grinding by conventional means is difficult and generally results in degradation of the heat stability of the polymer.
It has been proposed in U.S. Pat. No. 3,901,842, issued Aug. 26, 1975, to coagulate the vinyl polymer latex, centrifuge the same to form a polymer wet cake and then redisperse the wet cake by use of a redispersant, such as polyacrylate. Thereafter, the redispersed wet cake is sent to the spray dryer. However, this process still is not the final or desired answer. There is still a great need in the art to produce plastisol vinyl resins having improved properties by an economical process.